In response to a registered hall call, conventional elevator dispatching logic provides for audio or visual announcement of the assigned car in the hallway when the assigned car reaches a commitment point. The commitment point is defined as the location of the elevator at which it begins deceleration. The announcement is made no later than the commitment point because at that point the car must stop at the floor and there is not much advantage to delaying the announcement. In conventional logic, the announcement is made no sooner because an elevator dispatcher which controls the assignment of cars to hall calls uses the time between registration and announcement to make the best assignment. This announcement gives the passenger approximately two seconds notice before the car doors begin to open. An example of this conventional dispatching logic is the RSR scheme in "Relative System Response Elevator Call Assignments", U.S. Pat. No. 4,815,568.
A drawback of RSR is that while waiting for an elevator, a passenger naturally becomes anxious about which car will arrive, and the level of anxiety increases as the waiting time grows. This level of anxiety could be greatly reduced.
The Japanese elevator market requires the announcement of the car as soon as the waiting passenger registers his/her hall call. This feature is commonly referred to as ICA, or Instantaneous Car Assignment.
The problem with ICA is that often a car which appears to be an excellent candidate for a first assignment when the hall call is registered can become delayed by its assignment to hall calls and car calls entered after the first assignment. This can lead to a call that waits a very long time, which is more than 60 seconds. Assignment of the hall call, for example, may be to the car with the shortest Remaining Response Time (RRT). RRT is an estimation of the amount of time required for an elevator to reach the commitment point of the floor at which the hall call is registered, given the car calls and hall calls to which the elevator car is committed. Alternatively, Remaining Response Time may be defined as an estimation of the amount of time required for an elevator to reach the floor at which the hall call is registered, given the car calls and hall calls to which the elevator car is committed.
FIGS. 1-3 illustrate this problem. A group of six elevators serve 18 floors. As shown in FIG. 1, a down hall call was registered by a new passenger at Floor 12. The RRT for each car relative to this new hall car is shown above or below the car. The call becomes assigned to Car 3 because its RRT (Remaining Response Time) was lower than the other cars. Because the ICA feature is in effect, the assignment process is not repeated to determine if any assignment other than the initial one might be better.
At the time of the snapshot of the system in FIG. 1, car #3 had just cancelled an UP hall call on Floor 12 and was opening its doors at the moment when the new passenger registered the down hall call on Floor 12. An up-traveling passenger enters the car at Floor 12.
Because the up-traveling passenger had not yet registered the car call, the RRT for Car 3 relative to the new down hall call was only five seconds. Car 5 is loading new passengers on floor 16. It has an assigned hall call on floor 15 and has an RRT of 21 seconds for the new down hall call.
FIG. 2 shows the system after the down hall call has been waiting for 32 seconds. Instead of cancelling the assigned down hall call on floor 12, the assigned car (Car 3) travels toward floor 18 to fulfill its car call. Because of ICA, the down hall call on floor 12 must wait for Car 3 to return. Car 5 has bypassed Floor 12, and Car 6 is about to bypass Floor 12. Also, Car 4 is empty and traveling toward Floor 14 to reverse and answer a down hall call.
FIG. 3 shows the system after the down hall call at Floor 12 has been waiting for 67 seconds. Car 4 has already bypassed Floor 12. In the meantime, Car 3 is moving toward Floor 12 but still must make a car call stop on Floor 13.
These figures show that car #3 was initially judged to be a good assignment because of the very small RRT. In hindsight, any of three other cars (Cars 6, 5, or 4) would have reached Floor 12 sooner.